Device and process for measuring solid concentrations in liquids

ABSTRACT

In a method for measuring solid concentrations in liquids, the light from two differently modulated sources (Q1, Q2) directed on the liquid is measured at the same time and processed in a combined multi-beam measuring process consisting of a 90° light scatter (Q1, P, D4; Q2, P, D3) and back-scatter process (Q1, P1, D1; Q2, P3, D2) and the solid concentration is found therefrom. The process is suitable for the simultaneous measurement of both extremely low and extremely high solid concentrations by using a 90° light scatter process for low concentrations and a back scatter process for high concentrations. The multi-beam process which is based on two measurements through exactly the same window areas makes it possible effectively to compensate for undesired soiling of the window areas. Mechanical devices for cleaning the window areas and to eliminate interfering light scatter are proposed.

The present invention relates to a device and a process for measuringsolid concentrations in liquids, wherein the light from two sourcesdirected at the liquid is simultaneously measured in a combined 90°scattered-light and backscatter process and processed, and from this thesolid concentration is determined.

BACKGROUND AND SUMMARY OF THE INVENTION

In German Patent 3905101, a process and a device are described formeasuring concentrations of suspended particles, the process beingparticularly suited for high particle concentrations. In this processlight introduced into the suspension is scattered at the particles andmeasured at two locations at different distances. The particleconcentration is determined from a combination of these measured values.This is done on the assumption that the introduced light is coupled intoand out of the same window, which compensates for an eventual soiling ofthe window. The disadvantage of this is the fact that this assumption isonly correct in specific situations, and that inaccurate measurementsoccur because of the window with nonhomogeneous soiling.

Processes for measuring low and minimal particle concentrations areknown in water technology, where turbidity measurement is employed toassess the quality of drinking water, for example. Scattered-lightprocesses are predominantly used for this. At low concentrations theterm turbidity is generally used, whereas with high particleconcentrations the terms predominantly used are solid content or solidconcentrations. The term solid concentration (in g/l) will be usedexclusively below, even with extremely low solid concentrations, whereotherwise the term used would be turbidity values.

The object of the present invention is to disclose devices and processeswith which solid concentrations, particularly low as well as high, canbe determined with the same measuring process or the same measuringarray via the 90° scattered light and backscatter.

In accordance with the invention, this object is attained by means ofdevices and processes in accordance with the wording in claims 1 through12. Shown are:

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 a schematic representation of a 90° scattered-light process;

FIG. 2 a schematic representation of a backscatter process having twodetectors and two different light paths;

FIG. 3 a schematic representation of a multiple-beam process for 90°scattered light measurement having two sources and two detectors;

FIG. 4 a schematic representation of a multiple-beam process inaccordance with the invention for backscatter measurement having twosources and two detectors;

FIG. 5 a schematic representation of a combination in accordance withthe invention of the multiple-beam processes for 90° scattered lightmeasurement and 120° backscatter measurement having two sources and fourdetectors, seen from above;

FIG. 6 a schematic representation of a combination in accordance withthe invention of the multiple-beam processes for 90° scattered lightmeasurement and 120° backscatter measurement having two sources and fourdetectors, seen from the side;

FIG. 7 a schematic representation of an exemplary embodiment having 2sources and 4 detectors, seen from the side;

FIG. 8 the prism of FIG. 7 in a top view;

FIG. 9A the prism of FIG. 7 in view A;

FIG. 9B the prism of FIG. 7 in view B;

FIG. 10 a schematic representation of a second exemplary embodimenthaving a cleaning element and two screens, seen from the side; and

FIG. 11 a schematic representation of a third exemplary embodiment forthe arrangement of two sources and two detectors, seen from above.

The invention is described below in detail in conjunction with FIGS. 1through 11.

DETAILED DESCRIPTION

FIG. 1 shows a schematic representation of a known arrangement for a 90°scattered-light process.

Source Q1 transmits light through the window F onto the substance M tobe measured, which contains particles P and is then encompassed bywindows F1 and F2. The light from source Q1 is attenuated on the onehand by the substance M to be measured, and exits the substance throughthe window F1, after which it falls onto the detector D1, whichcorresponds to a transmission measurement in accordance with formula(1.1):

    I.sub.1 /I.sub.D11, =exp(cεd)/(A.sub.F A.sub.F1)   (1.1)

where

I_(i) =light intensity of source i

I_(Dji) =light intensity of source i at detector j

c=solid concentration

α.sub.γ =scatter coefficient at angle γ

ε=absorption coefficient

A_(Fi) =absorption factor in window F_(i)

d=light path in substance to be measured

d_(i) =i^(th) light path in substance to be measured

The light of source Q1 is scattered on the other hand at particle P,whereupon it exits the substance to be measured at an angle of 90°through window F2 and falls onto detector D2, which corresponds to ascattered light measurement in accordance with formula (1.2):

    I.sub.1 /I.sub.D21 =exp(cεd)/(α.sub.π c A.sub.F A.sub.F2)(1.2)

Formulas (1.1) and (1.2) are applicable, ignoring multiple scatteringsin the substance to be measured.

FIG. 2 schematically shows a known arrangement for a backscatter processhaving two detectors.

Source Q1 transmits light through the window F onto the substance M tobe measured, which contains particles P1 and P2 and is encompassed bywindows F, F1 and F2. The light is scattered at particles P1 and P2, andthe corresponding scattered light exits the substance to be measured atan angle γ of 120°, for example, through windows F1 and F2, and fallsupon the corresponding detectors D1 and D2, which corresponds toscattered light measurements in accordance with formulas (2.1) and(2.2):

    I.sub.1 /I.sub.D11 =exp(cεd.sub.1)/(α.sub.120 c A.sub.F1) (2.1)

    I.sub.1 /I.sub.D21 =exp(cεd.sub.2)/(α.sub.120 c A.sub.F A.sub.F2)                                                 (2.2)

Formulas (2.1) and (2.2) are applicable, ignoring multiple scatteringsin the substance to be measured.

FIG. 3 shows a schematic representation of a multiple-beam measurementprocess for a 90° scattered light measurement having two sources and twodetectors.

In this case sources Q1 and Q2 transmit differently modulated light.This light is detected with detectors D3 and D4, after interaction withparticles P contained in the substance M to be measured. Source axes QA1and QA2 of sources Q1 and Q2 are perpendicular to one another. Thesubstance to be measured is encompassed by windows F1°, F2°, F3 and F4.Detectors D3 and D4 serve both to detect the transmitted light of therespective source located opposite and to detect the 90° scattered lightof the source disposed perpendicular thereto. Ignoring multiplescatterings, formulas (3.1) and (3.2) apply for transmitted lightmeasurement, and formulas (3.3) and (3.4) apply for the scattered lightmeasurement:

    I.sub.1 /I.sub.D31 =exp(cεd)/(A.sub.F1* A.sub.F3)  (3.1)

    I.sub.2 /I.sub.D42 =exp(cεd)/(A.sub.F2* A.sub.F4)  (3.2)

    I.sub.1 /I.sub.D41 =exp(cεd)/(α.sub.90 c A.sub.F1* A.sub.F4) (3.3)

    I.sub.2 /I.sub.D32 =exp(cεd)/(α.sub.90 c A.sub.F2* A.sub.F3) (3.4)

Sources Q1 and Q2 and detectors D1 and D2 are disposed such that thecorresponding light paths d are each of identical size.

In this way four measured values are obtained, from which the followingrelationships are formed:

    I.sub.D41 /I.sub.D31 =α.sub.90 A.sub.F4 /A.sub.F3    (3.5)

    I.sub.D32 /I.sub.D42 =α.sub.90 c A.sub.F3 /A.sub.F4  (3.6)

Formulas (3.5) and (3.6) are independent of source intensities I₁ and I₂and of the damping exp(cεd) in the substance to be measured.

If sources Q1 and Q2 are controlled such that condition (3.7) is met,then multiplication of formula (3.5) with (3.6) yields the result (3.8):

    I.sub.D31 =I.sub.D42 =k                                    (3.7 )

    I.sub.D32 I.sub.D41 =(α.sub.90 c).sup.2 k.sup.2      (3.8)

where the measured light intensities I_(D32) and I_(D41) are nowdependent only on the scatter coefficient α₉₀, a constant k and thesolid concentration c.

FIG. 4 shows a schematic representation of a multiple-beam process ofthe invention for a backscatter measurement having two sources and twodetectors.

In this instance sources Q1 and Q2 transmit differently modulated light,which is coupled into the substance M to be measured through windows F1°and F2°. After interaction with particles P1, P2, P3 and P4 contained inthe substance M to be measured, the light is detected with detectors D1and D2. The substance to be measured is encompassed by windows F1, F1°,F2 and F2°, which are located in the same window surface, although thisis not absolutely necessary. Detectors D1 and D2 serve to detect thebackscatter at an angle γ of 135°. The following light paths result fromthis: d₁ from F1°-P1-F1, d₂ from F2°-P2-F1, d₃ from F2°-P3-F2 and d₄from F1°-P4-F2. In particular, light paths d₁ and d₃, and d₂ and d₄,respectively, can be of equal size. The following formulas include thispresupposition. Ignoring multiple scatterings, formulas (4.1) and (4.2)apply for backscatter measurement for source Q1, and formulas (4.3) and(4.4) apply for source Q2:

    I.sub.1 /I.sub.D11 =exp(cεd.sub.1)/(α.sub.135 A.sub.F1* A.sub.F1)                                                 (4.1)

    I.sub.1 /I.sub.D21 =exp(cεd.sub.2)/(α.sub.135 A.sub.F1* A.sub.F2)                                                 (4.2)

    I.sub.2 /I.sub.D22 =exp(cεd.sub.1)/(α.sub.135 c A.sub.F2* A.sub.F2)                                                 (4.3)

    I.sub.2 /I.sub.D12 =exp(cεd.sub.2)/(α.sub.135 c A.sub.F2* A.sub.F1)                                                 (4.4)

In this way 4 measured values are obtained, from which the followingrelationships (4.5) and (4.6) are formed:

    I.sub.D11 /I.sub.D21 =exp[cε(d.sub.2 -d.sub.1)]A.sub.F1 /A.sub.F2 ( 4.5)

    I.sub.D22 /I.sub.D12 =exp[cε(d.sub.2 -d.sub.1)A.sub.F2 /A.sub.F1 (4.6)

Formulas (4.5) and (4.6) are independent of source intensities I₁ and I₂and of the scatter coefficient α₁₃₅ in the substance to be measured.

If sources Q1 and Q2 are controlled such that the condition (4.7) ismet, then the result (4.8) is obtained through the multiplication offormula (4.5) with (4.6):

    I.sub.D21 =I.sub.D12 =k                                    (4.7)

    I.sub.D11 I.sub.D22 =k.sup.2 exp[2cε(d.sub.2 -d.sub.1)](4.8)

where the measured light intensities I_(D11) and I_(D22) are nowdependent only on the difference between the light paths d₁ and d₂,which are constant; on the absorption coefficient ε; on a constant k;and on the solid concentration c.

By means of a skillful spatial arrangement, the two measuring processesin FIG. 3 and FIG. 4 can be combined so that the same sources Q1 and Q2can be used. The electronics required for evaluation are practicallyidentical in the two measuring processes.

FIG. 5 shows a schematic representation of a combination in accordancewith the invention of the multiple-beam processes for a 90° scatteredlight measurement and a 120° backscatter measurement having two sourcesand four detectors, seen from above.

In this arrangement the multiple-beam process for a 90° scattered lightmeasurement comprises sources Q1 and Q2 with corresponding source axesQA1 and QA2, particles P or P', detectors D3 and D4 with correspondingdetector axes DA3 and DA4, windows F1°, F2°, F3 and F4, and substance Mto be measured.

The light from the source Q1 is carried to particle P of the substance Mto be measured. The light from the source Q2 is carried to particle P'of the substance M to be measured.

At least approximately, and more preferably exactly, the detector axisDA3 of the detector D3 coincides with source axis QA1. Detector D3detects the transmitted light of formula (5.1), at least approximately,and in more preferably exactly, the detector axis DA4 of detector D4coincides with source axis QA2. The detector D4 detects the transmittedlight in accordance with formula (5.2):

    I.sub.1 /I.sub.D31 =exp(cεd)/(A.sub.F1* A.sub.F3)  (5.1)

    I.sub.2 /I.sub.D42 =exp(cεd)/(A.sub.F2* A.sub.F4)  (5.2)

The detector axis DA4 of detector D4 is perpendicular to the source axisQA1. Detector D4 detects the scattered light from the particle P inaccordance with formula (5.3); the detector axis DA3 of detector D3 isperpendicular to source axis QA2. Detector D3 detects the scatteredlight from the particle P' in accordance with formula (5.4):

    I.sub.1 /I.sub.D41 =exp(cεd)/(α.sub.90 c A.sub.F1* A.sub.F4) (5.3)

    I.sub.2 /I.sub.D32 =exp(cεd)/(α.sub.90 c A.sub.F2* A.sub.F3) (5.4)

The vertical plane in which QA1 lies is disposed parallel to thevertical plane in which QA2 lies and nearly coincides with it. Inparticular, the vertical planes can coincide. The opening angle of D3 issuch that it permits both a transmitted light measurement of Q1 and ascattered light measurement of the particle P'; the opening angle of D4is such that it permits both a transmitted light measurement of Q2 and ascattered light measurement of the particle P.

In this arrangement, the multiple-beam process for a backscattermeasurement comprises sources Q1 and Q2 with corresponding source axesQA1 and QA2; particles P1 or P2, P3 or P4, respectively; detectors D1and D2 with corresponding detector axes DA1 and DA2; windows F1°, F2°,F1 and F2; and the substance M to be measured.

Detector axis DA1 of detector D1 is disposed such that detector D1 candetect the backscatter of the particle P1 in accordance with formula(5.5) and the backscatter of the particle P2 in accordance with formula(5.6) at an angle of 120°; detector axis DA2 of detector D2 is disposedsuch that the detector D2 can detect the backscatter of the particle P3in accordance with formula (5.7) and the backscatter of the particle P4in accordance with formula (5.8) at an angle of 120°:

    I.sub.1 /I.sub.D11 =exp(cεd.sub.1)/(α.sub.120 A.sub.F1* A.sub.F1)                                                 (5.5)

    I.sub.1 /I.sub.D12 =exp(cεd.sub.2)/(α.sub.120 A.sub.F2* A.sub.F1)                                                 (5.6)

    I.sub.2 /I.sub.D22 =exp(cεd.sub.1)/(α.sub.120 c A.sub.F2* A.sub.F2)                                                 (5.7)

    I.sub.2 /I.sub.D21 =exp(cεd.sub.2)/(α.sub.120 c A.sub.F1* A.sub.F2)                                                 (5.8)

By means of a corresponding arrangement, it is ensured that on the onehand the two light paths d₁, namely F1°-P1-F1, and d₃, namely F2°-P3-F2,in the substance M to be measured are identical in size and, on theother hand, that the two light paths d₂, namely F2°-P2-F1, and d₄,namely F1°-P4-F2, in the substance M to be measured are identical insize. In this case d₁ is the shorter light path, and d₂ is the longerlight path.

Relationships (5.1) to (5.3) and (5.2) to (5.4) are formed in pairs fromthe 4 measured values in accordance with formulas (5.1) through (5.4),resulting in formulas (5.9) and (5.10):

    I.sub.D41 /I.sub.D31 =α.sub.90 c A.sub.F4 /A.sub.F3  (5.9)

    I.sub.D32 /I.sub.D42 =α.sub.90 c A.sub.F3 /A.sub.F4  (5.10)

Formulas (5.9) and (5.10) are likewise independent of source intensitiesI₁ and I₂ and of the damping exp(cεd) in the substance to be measured.

If sources Q1 and Q2 are controlled such that condition (5.11) is met,then the result (5.12) is obtained through the multiplication of formula(5.9) with (5.10):

    I.sub.D31 =I.sub.D42 =k                                    (5.11)

    I.sub.D41 I.sub.D32 =(α.sub.90 c).sup.2 k.sup.2      (5.12)

where the measured light intensities I_(D41) and I_(D32) are nowdependent only on the scattering coefficient α₉₀, a constant k and thesolid concentration c.

Relationships (5.8) to (5.5) and (5.6) to (5.7) are formed in pairs fromthe 4 measured values in accordance with formulas (5.5) through (5.8),resulting in formulas (5.13) and (5.14):

    I.sub.D11 /I.sub.D21 =exp [cε(d.sub.2 -d.sub.1)] A.sub.F1 /A.sub.F2 (5.13)

    I.sub.D22 /I.sub.D12 =exp[cε(d.sub.2 -d.sub.1)] A.sub.F2 /A.sub.F1 (5.14)

Formulas (5.13) and (5.14) are independent of source intensities I₁ andI₂ and of the scattering coefficient α₁₂₀ in the substance to bemeasured.

If sources Q1 and Q2 are-controlled such that condition (5.15) is met,then the result (5.16) is obtained through the multiplication of formula(5.13) with (5.14):

    I.sub.D21 =I.sub.D12 =k                                    (5.15)

    I.sub.D11 I.sub.D22 =k.sup.2 exp[2cε(d.sub.2 -d.sub.1) (5.16)

where the measured light intensities I_(D11) and I_(D22) are nowdependent only on the difference between light paths d₁ and d₂, whichare constant; on the absorption coefficient ε; on a constant k; and onthe solid concentration c. Formulas (5.1) through (5.16) apply with theomission of multiple scatterings.

By means of an electrical trigger circuit, light intensities I₁ and I₂are generated in light emitting diodes, for example in GaAs lightemitting diodes, GaAlAs light emitting diodes or laser diodes andabsorbed into the substance M to be measured. Afterward lightintensities I_(D11), I_(DI2), I_(D21), I_(D22), I_(D31), I_(D32),I_(D41) and I_(D42) are measured, which are converted into electricalsignals in silicon photodiodes, PIN diodes, avalanche diodes orphotomultipliers. The signals are supplied to a signal processing means,by means of which the solid concentrations in the substance to bemeasured are ascertained.

Detectors D1-D4 each detect light from both sources Q1 and Q2. If thesources are operated with differently modulated light, the two portionscan be separated in the detector signals in a known way. Types ofmodulation that come into consideration are the use of variation inlight intensity having a pulse-duty factor of 50%, pulse operationhaving a pulse-duty factor much smaller than 50%, and other known typesof modulation.

It is known that the conversion characteristic of the optoelectroniccomponents is subject to aging and is very temperature-dependent. Thesedependencies, as well as the dependency on the soiling of windows F1°,F2°, F1, F2, F3 and F4, are compensated for by means of themultiple-beam principle.

The distance between the two vertical planes in which source axes QA1,QA2, which are perpendicular to one another, are located is dependent onlight paths d₁ and d₂ and on the angle γ of the backscatter measurement.

By means of the special arrangement of the two source axes QA1 and QA2with respect to detector axes DA1/-DA4, it is possible to execute the90° scattered light process and the backscatter process as multiple-beamprocesses with only two sources, which proves to be particularlyadvantageous.

FIG. 6 shows a schematic representation of a combination in accordancewith the invention of the multiple-beam processes for a 90° scatteredlight measurement and a backscatter measurement having two sources andfour detectors, seen from the side.

In this figure, reference numerals Q1, Q2, QA1, QA2, D1-D4, DA1-DA4, M,P, P1, P2, P3, P4, F1°, F2°, F1-F4 correspond to those in FIG. 5. Themode of operation has already been described in detail in FIG. 5.

FIG. 7 shows a schematic representation of an exemplary embodimenthaving 2 sources and 4 detectors, seen from the side.

A prism PR, via which the light from sources Q1 and Q2 is coupled in, ismounted on the housing G. Two condenser lenses K1 and K2 are mounted onthe housing G, and the transmitted light from sources Q1 and Q2 and the90° scattered light is supplied to detectors D3 and D4, which arelocated behind the condenser lenses, after interaction with thesubstance M to be measured, which is located in the housing. Along withsources Q1 and Q2, two further detectors D1 and D2 that serve to measurebackscatter are mounted at the prism.

The housing G can be made of black-anodized aluminum, for example;materials such as sapphire, quartz or conventional glass are possiblefor condenser lenses K1 and K2. The condenser lenses are glued in thehousing.

Two model SFH 414 (Siemens) light emitting diodes, with which shortlight pulses are generated are used as sources Q1 and Q2. The repetitionrate is 1 kHz, with a pulse-duty factor of 5%, a light wavelength of 950nm and a projection angle of +/- 11°. By means of prism PR, which willbe described in detail below, the beams are coupled into the substanceto be measured in the housing at an angle of 45°. Source axes QA1 andQA2 are perpendicular to one another, and the vertical planes in whichthey lie are spaced apart by 2.8 mm. Source axes QA1 and QA2, from adistance of 40 mm, strike the respective opposed condenser lenses K1 andK2, which have a diameter of 19 mm. Detectors D3 and D4 are mounted nearthe focal point, behind the condenser lenses. The transmitted light fromQ1 and the 90° scattered light are measured with D3, and the transmittedlight from Q2 and the 90° scattered light from Q1 are measured with D4.SFH 2030 (Siemens) silicon photodiodes are used as detectors D3 and D4.The photocurrents of these diodes are processed in an electroniccircuit, and the solid concentration c is calculated from the measuringresults by means of formulas (5.11) and (5.12), taking intoconsideration the scatter coefficient α₉₀.

As the solid concentration c rises, the transmitted light becomesincreasingly weaker and the accuracy of measurement drops. Above a limitdependent on the substance to be measured, the solid concentration isadvantageously calculated from the backscattered light.

By means of the same prism PR that is used for coupling in light, thebackscatter is coupled out at two locations located opposite oneanother, at a 45° angle; in the direction of detector axes DA1 and DA2.The directions of the two outcouplings are at a 120° angle to sourceaxes QA1 and QA2. Detectors D1 and D2 are located below the outcouplinglocations. The light paths in the substance to be measured from sourceQ1 to the detector D1 and from source Q2 to the detector D2 have anaverage length of 8 mm; the light paths in the substance to be measuredfrom source Q2 to the detector D1 and from source Q1 to detector D2 havean average length of 16 mm. SFH 2030 silicon photodiodes are likewiseused as detectors D1 and D2.

The photocurrents of these diodes are processed in an electroniccircuit, and the solid concentration c is calculated from the measuredresults by means of formulas (5.15) and (5.16), taking intoconsideration the absorption coefficient ε.

FIG. 8 shows the prism of FIG. 7 in a top view. The prism isadvantageously made of sapphire that has a refraction index of 1.76, butcan also be made of quartz that has a refraction index of 1.45, orconventional glass. The prism is glued inside the housing.

FIG. 9A and FIG. 9B show the prism of FIG. 7 in view A and view B,respectively.

FIG. 10 shows a second exemplary embodiment having a cleaning elementand two screens, shown from the side.

No measuring errors result from soiling of the optical components,because these errors are compensated for by the multiple-beam process;however, the total quantity of light that can be evaluated decreases,and this lowers the resolution capability of the measuring array.

A cleaning element R, in which recesses A1, A2 and A3 are provided, islocated in housing G, to which condenser lenses K1 and K2 and prism PRare secured. In the resting state, that is, when the cleaning element isinactive, these recesses are located precisely above the prism andcondenser lenses. The cleaning element is displaceably disposed for amovement perpendicular to source axes QA1 and QA2. The element can beprovided with a pneumatic drive, for example. The movement of thecleaning element is arranged such that it sweeps over at least theentire surface of the prism and condenser lenses. Afterwards thecleaning element is returned to its resting state. Depending on the typeand concentration of the substance to be measured, the necessity arisesat various time intervals to eliminate the soiling on the prism andcondenser lenses by activating the cleaning element. The cleaningelement can be made of black-colored plastic or hard rubber, forexample. Screens B1 and B2, which are used to screen out parasiticscattered light, are disposed on cleaning element R. Source Q2, with itssource axis QA2 and the boundaries l₂ of the light cone, is aimed at thecondenser lens K2. Parasitic scattered light can arise under thefollowing conditions, for example, and can be eliminated by means ofscreen B1: after passing through the prism, the light from source Q1,which with its source axis QA1 is aimed at the condenser lens K1,strikes a particle or scratch at location P on the prism surface, andbecause of this is scattered in different directions, but particularlyin the direction of condenser lens K2, as is indicated by a scatteredlight beam SS1. If a particle or scratch is located at location P' onthe condenser lens K2, then further scattering occurs, after which inparticular a scattered light beam SS2 causes a quantity of parasiticscattered light on the detector located behind the condenser lens K2.The screen B1 is disposed such that it screens out the scattered lightbeams SS1 before they strike the condenser lens K2, but withoutaffecting the light cone of source Q2. The screen B1 can be mounted onthe cleaning element R, or be embodied as an integrated component ofcleaning element R.

FIG. 11 shows a schematic representation of a third exemplary embodimentfor the arrangement of two sources and two detectors in the housing,seen from above.

A conical bore whose opening angle is 90 degrees, for example, isdisposed perpendicular to the housing surface and has a center Z1 andZ2. Bores having centers ZQ1, ZQ2, ZD1 and ZD2 and at an angle of 90degrees, for example, from the cone surface, are provided for sourceaxes QA1 and QA2 and detectors DA1 and DA2. This creates four recesses,in which the windows, for example made of sapphire, are glued, behindwhich sources Q1 and Q2 and detectors D1 and D2 are located. Thisarrangement meets the conditions for backscatter measurement.

With such measuring devices according to the invention as shown in FIGS.5-11, a broader measuring range for fresh water can be detected, rangingfrom the slightest contamination, for example a dissolution of 0.1 NTU,to sludge, for example with solid concentrations up to 200 g/l, andcontinuous measurement over the entire range is possible.

If only high solid concentrations are measured, then the arrangement ofdetectors D3 and D4, and condenser lenses K1 and K2, can be omitted;that is, the 90° scattered light measurement can be omitted.

Applications for a device of this type and associated processes are themeasurement of solid concentrations in water technology, waste watertechnology, particularly in sewage processing systems, where very highsludge concentrations must be dealt with; foodstuff technology, forexample the production of fruit concentrates, the pharmaceuticalindustry; and in the area of biotechnology, particularly to determinebiomass, in the determination of growth rates in cell cultures.

It is essential to the invention that solid concentrations, particularlylow as well as high, can be detected in a single measuring array with amultiple-beam process using a combined measuring process that comprises,on the one hand, a 90° scattered-light process and, on the other hand, abackscatter process; only two sources are used, and the correspondingsignals are evaluated with the same electronics. The multiple-beamprocess, which is based on 2 measurements through exactly the samewindow surface, permits effective compensation for undesired soiling ofthe window surfaces and aging of the optoelectronic components.

The foregoing has described the preferred principles, embodiments andmodes of operation of the present invention; however, the inventionshould not be construed as limited to the particular embodimentsdiscussed. Instead, the above-described embodiments should be regardedas illustrative rather than restrictive, and it should be appreciatedthat variations, changes and equivalents may be made by others withoutdeparting from the scope of the present invention as defined by thefollowing claims.

What is claimed is:
 1. A device for measuring solid concentrations inliquids, comprising:a housing for containing a substance to be measured,having windows to allow the passage of light into and out of thehousing; two light sources mounted on the housing, each light sourcegenerating a light cone having a source axis that projects into thehousing: and at least two fight detectors mounted on the housing, eachdetector having a detector axis that projects into the housing; whereinthe source axes of the light cones are at least substantiallyperpendicular to one another and lie in at least substantially parallelplanes; wherein a first detector axis is disposed to intersect a firstsource axis at a first point with a first light path d₁ in the substanceto be measured, and to intersect a second source axis at a second pointwith a second light path d₂ in the substance to be measured, the secondlight path d₂ being longer than the first light path d₁ ; and wherein asecond detector axis is disposed to intersect the second source axis ata third point with a light path d₃ in the substance to be measured andto intersect the first source axis at a fourth point with a light pathd₄ in the substance to be measured, the fourth light path d₄ beinglonger than the third light path d₃.
 2. A device in accordance withclaim 1, further comprising a third detector mounted on the housing andhaving a third detector axis substantially in alignment with the firstsource axis, and a fourth detector mounted on the housing and having afourth detector axis substantially in alignment with the second sourceaxis.
 3. A device in accordance with claim 2, wherein a ratio of thelength of the light path, in the substance to be measured, from thefirst source to the third detector to the length of the light path, inthe substance to be measured, from the first source to the firstdetector, and the ratio of the length of the light path, in thesubstance to be measured, from the second source to the fourth detectorto the length of the light path, in the substance to be measured, fromthe second source (Q2) to the detector, is in a range between 1 and 10.4. A device in accordance with claim 3, wherein the ratio is
 5. 5. Adevice in accordance with claim 1, wherein the first and second sourceaxes and the first and second detector axes are disposed to project intothe housing through windows disposed in conically shaped portions of thehousing.
 6. A device in accordance with claim 1, further comprising aprism mounted on the housing and disposed so that the source axes anddetector axes are covered upon entrance into the housing, the prismbeing flat on at least on one side.
 7. A device in accordance with claim1, wherein the light sources comprise light emitting diodes, selectedfrom the group consisting of GaAs diodes, GaAlAs diodes and laserdiodes.
 8. A device in accordance with claim 7, wherein the detectorscomprise a device selected from the group consisting of silicon PINdiodes, avalanche diodes and photomultipliers.
 9. A device in accordancewith claim 1, wherein a ratio of the length of the light path, in thesubstance to be measured, from the first source to the first detector tothe length of the light path, in the substance to be measured, from thefirst source to the second detector, and a ratio of the length of thelight path, in the substance to be measured, from the second source tothe second detector to the length of the light path, in the substance tobe measured, from the second source to the first detector, is in a rangeof less than
 1. 10. A device in accordance with claim 9, wherein theratio is 0.5.
 11. A device in accordance with claim 1, furthercomprising a device for cleaning the windows comprising a displaceablyseated, piston-shaped cleaning element.
 12. A device in accordance withclaim 1, further comprising means disposed on the cleaning element toscreen out parasitic scattered light.
 13. A device in accordance withclaim 1, wherein the source axes of the light cones generated by thelight sources lie in the same plane.
 14. A process in accordance withclaim 1, further comprising the step of providing a substance to bemeasured, wherein the solid concentration measured determines thebiomass of the substance.
 15. A process for measuring solidconcentrations in liquids in a device having a housing for a substanceto be measured, a first and second light source each for generatinglight on an axis into the housing, and a first and second detector fordetecting light exiting the housing, wherein a first detector axis isdisposed to intersect a first source axis at a first point with a firstlight path d₁ in the substance to be measured, and to intersect a secondsource axis at a second point with a second light path d₂ in thesubstance to be measured, the Second light path d₂ being longer than thefirst light path d₁, and wherein a second detector axis is disposed tointersect the second source axis at a third point with a light path d₃in the substance to be measured and to intersect the first source axisat a fourth point with a light path d₄ in the substance to be measured,the fourth light path d₄ being longer than the third light path d₃,comprising the steps of:modulating differently the light sources;measuring the backscatter intensities I_(D11) and I_(D) ₂₁ in light pathd₁ and backscatter intensifies I_(D12) and I_(D21) in light path d₂ ;determining ratios I_(D11) /I_(D21) and I_(D22) /I_(D12) ; controllingthe light sources so that I_(D21) equals I_(D12) equals k; convertingthe backscatter intensities I_(D11) and I_(D22) into electrical signals;and detecting the solid concentration c in accordance with themathematical relation

    I.sub.D11 I.sub.D22 =k.sup.2 exp(2cε(d.sub.2 -d.sub.1))

wherein the variable I_(Dji) indicates the light intensity of source iat detector j; k indicates a constant; and ε indicates the absorptioncoefficient.
 16. A process in accordance with claim 15 wherein theapparatus includes a third detector having a third detector axis alignedwith the first source axis and perpendicular to the second source axisand a fourth detector having a fourth detector axis aligned with thesecond source axis and perpendicular to the first source axis, themethod further comprising the steps of:measuring the transmitted lightintensities I_(D31) of a light path from the first source to the thirddetector and I_(D42) of a light path from the second source to thefourth detector and the 90° scattered light intensities I_(D41) of alight path from the first source to the fourth detector and I_(D32) of alight path from the second source to the third detector; calculating theratios I_(D41) /I_(D31) and I_(D32) /I_(D42) ; controlling the sourcesso that I_(D31) equals I_(D42) equals k; converting the 90° scatteredlight intensities I_(D41) and I_(D32) into electrical signals; anddetermining the solid concentration c for low solid concentrations inaccordance with the mathematical relation

    I.sub.D41 I.sub.D32 =(α.sub.90 c).sup.2 k.sup.2 ;

and for high solid concentrations in accordance with the mathematicalrelation

    I.sub.D11 I.sub.D22 =k.sup.2 exp(2cε(d.sub.2 -d.sub.1))

and wherein α₉₀ indicates the scatter coefficient of the 90° scatteredlight measurement.
 17. A process in accordance with claim 15 formeasuring solid concentrations in water technology further comprisingthe step of providing a substance to be measured from the groupconsisting of waste water treatment products, sewage processing systemsproducts; foodstuff processing products; pharmaceutical processingproducts; and biotechnology processing.